In mechanical design, friction must be reduced by lubricating rotary members in an oil bath, and numerous applications call for immersing portions of the rotary members in the oil bath, while other portions, adjacent to them, operate in a sealed, oil-free environment. To do this, seal assemblies are known, as described for example in U.S. Pat. No. 5,348,312, which are housed inside the gap defined between a normally fixed, hollow casing and a rotary member housed inside the casing, and which, in use, seal the gap to prevent oil flow into the dry environment.
More specifically, seal assemblies of the above type are axially symmetrical with respect to the axis of the rotary member to which they are fitted, and comprise a metal inner ring integral with the rotary member; a metal outer ring fixed to the casing; and sealing members made of elastically deformable polymer material and interposed between the inner ring and the rotary member, between the outer ring and the casing, and between the rings themselves.
More specifically, the inner ring comprises an axial portion fitted to the inner ring; and two radial portions extending from respective opposite ends of the axial portion and inwards of the gap between the casing and the rotary member. In the example described in U.S. Pat. No. 5,348,312, the radial portion facing the oil bath is of a larger extension than the other radial portion, and terminates close to the casing.
In axial half-section, the outer ring is substantially L-shaped, and comprises an axial portion, and a radial portion extending from one end of the axial portion. More specifically, the radial portion of the outer ring is adjacent to the smaller-extension radial portion of the inner ring, and defines, with the other radial portion of the inner ring, a sealing chamber between the two rings.
The sealing members comprise a first seal pressed between the axial portion of the inner ring and the rotary member; and a second seal completely covering the outer ring and interposed between the outer ring and the casing to seal the chamber between the two rings.
More specifically, on the side facing the oil bath, the second seal has a lip which provides for contact sealing in contact with the larger-extension radial portion of the rotary inner ring.
The above contact seal has the typical drawback of producing rapid wear of the lip of elastomeric material.
To at least partly eliminate the above drawback, the lip has a number of concentric, radially equally spaced, circumferential grooves to reduce the contact surface and, therefore, friction with the rotary inner ring. Though it provides for prolonging the working life of the seal with the sealing lip, the above solution is not altogether satisfactory.
Seal assemblies are also known, as illustrated for example in U.S. Pat. No. 4,974,869, in which the sealing lip has no grooves, and the radial portion of the metal inner ring has a single spiral groove. In actual use, the sealing lip engages the spiral groove rotating integrally with the rotary member, thus again providing for contact sealing. Moreover, as the inner ring rotates integrally with the rotary member, the spiral groove produces a fluid-dynamic “pumping” effect of the air between the lip and the inner ring, which forces the oil towards the oil environment, thus enhancing sealing performance.
In this case too, extensive sliding contact between the spiral groove on the inner ring and the sealing lip of elastomeric material produces severe mechanical wear, and so greatly reduces the working life, of the lip. Moreover, the thinness of the inner ring and the technical difficulties encountered in machining it prevent the formation of deeper grooves shaped to vary airflow, so that the working life of the seal assembly and the effectiveness of the fluid-dynamic “pumping” effect remain limited.